In the context of sustainable development, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as the present TRIP steels in order to reduce the fuel consumption and emission of greenhouse gas. These multiphase steels contain some retained austenite, a ductile phase that can transform into hard and brittle martensite during a mechanical solicitation. One the one hand, this transformation improves the mechanical properties during plasticity by bringing about an additional work-hardening. On the other hand, the appearence of a hard and brittle phase can give rise to premature cracking after necking. Knowing the good influence of martensitic transformation on the work-hardening, this Thesis starts with the characterisation of the relationship between transformation rates and testing conditions. It appears that, for each testing condition, there is an optimum austenite stability that leads to a maximum uniform strain. After necking under monotonic loading conditions, the damage mechanisms that takes place in these steels has been characterised. It can be concluded that the TRIP-aided steels that present low or moderate austenite stability behave exactly like Dual-Phase steels, in which martensite replaces retained austenite. However, a very stable retained austenite brings about a significant toughness improvement by providing an additional work-hardening contribution in the necking zone. The mechanical behaviour of these steels has also been characterised under cyclic loading conditions. The results indicate that, for particular loading conditions (i.e. low load levels), the martensitic transformation improves the fatigue properties.

<p>Case hardening distortions are a major problem for gear manufacturers. The aim of the current work is to create a simulation model, able to predict how and when case hardening distortions arise. The results presented in this thesis form a basis for such a model.</p><p>Two case hardening steels, with base carbon contents of 0.20 and 0.21 % C were studied using dilatometer experiments. One of them was carburized to 0.36, 0.52 and 0.65 % C in order to investigate the influence of carbon content. Experiments were performed during both isothermal and continuous heating and cooling conditions. The results were used to evaluate phase transformations, heat expansion behaviors and phase transformation strains. The expansion behavior of the material was modeled as a function of temperature, carbon content and phase fractions. The phase transformations to martensite and bainite were modeled, using the Koistinen-Marburger equation and a transformation rate equation based on Austin-Rickett kinetics, respectively. Experiments were simulated using the COMSOL Multiphysics software, to verify the model with respect to martensite and bainite transformations, heat expansion behavior and phase transformation strains.</p>

The crystal structure of metarossite, ideally CaV2O6 center dot 2H(2)O [chemical name: calcium divanadium(V) hexaoxide dihydrate], was first determined using precession photographs, with fixed isotropic displacement parameters and without locating the positions of the H atoms, leading to a reliability factor R = 0.11 [Kelsey & Barnes (1960). Can. Mineral. 6, 448- 466]. This communication reports a structure redetermination of this mineral on the basis of single- crystal X- ray diffraction data of a natural sample from the Blue Cap mine, San Juan County, Utah, USA (R1 = 0.036). Our study not only confirms the structural topology reported in the previous study, but also makes possible the refinement of all non- H atoms with anisotropic displacement parameters and all H atoms located. The metarossite structure is characterized by chains of edge- sharing [CaO8] polyhedra parallel to [100] that are themselves connected by chains of alternating [VO5] trigonal bipyramids parallel to [010]. The two H2O molecules are bonded to Ca. Analysis of the displacement parameters show that the [VO5] chains librate around [010]. In addition, we measured the Raman spectrum of metarossite and compared it with IR and Raman data previously reported. Moreover, heating of metarossite led to a loss of water, which results in a transformation to the brannerite- type structure, CaV2O6, implying a possible dehydration pathway for the compounds M2+V2O6 center dot xH(2)O, with M = Cu, Cd, Mg or Mn, and x = 2 or 4.

The overarching goal of this research was to generate a menu of shape memory alloys (SMAs) actuator materials capable of meeting the demands of aerospace applications. Material requirements were recognized to meet the demand for high temperature SMAs with actuating temperatures above 85 °C and provide material options capable of performing over 100K actuation cycles. The first study is a preliminary characterization for the down selection of Ni-rich NiTiHf15 compositions chosen for a more in-depth examination of the nano-precipitation and evolution of the H-phase. To make this selection, the effect of Ni content in Ni-rich NiTiHf high temperature shape memory alloys (HTSMAs) on processability, microstructure, and hardness was analyzed for three compositions (Ni50.1TiHf15, Ni50.3TiHf15, Ni50.5TiHf15). Each composition was characterized under three conditions: homogenized, 25%, and 50% thickness reduction through hot-rolling. The second study emphasized the processing and aging response of an industrially produced, hot-extruded Ni50.3Ti29.7Hf20 (at%) HTSMA. The samples were sectioned into two halves with half remaining as-extruded and the other half hot-rolled to a 25% reduction in thickness. A portion of both conditions underwent conventional aging for 3 hours at various temperatures ranging from 450-750 °C, and the other portion was pre-aged for 12 hours at 300 °C followed by conventional aging treatments. After processing, the samples were characterized by differential scanning calorimetry (DSC), Vickers hardness (HV) testing, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron radiation X-ray diffraction (SR-XRD). The relationship between the introduction of texturing, pre-aging, and aging on Ni-rich and high Hf-content compositions was investigated.

Stable and metastable phases of Fe and Al and structural anomalies of Zn and Cd have been studied by epitaxial Bain path (EBP) and minimum path (MNP) first-principles procedures, based on finding equilibrium structures from minimizing the Gibbs free energy G with respect to structure at a given hydrostatic pressure p and temperature T . The main accomplishments are as follows. (1) This dissertation illustrates the effectiveness of the MNP procedure for finding stable and metastable phases of an element by studying four Bravais phases of Fe including body-centered tetragonal (bct), body-centered cubic (bcc), face-centered cubic (fcc) and rhombohedral (rh) phases. The determination of absolute stability using MNP is new; MNP finds all the elastic constants cjj of a given state and the eigenvalues of the elastic constants matrix, which determine the absolute stability of the state. / (2) We have extended our search for stable and metastable phases from zero temperature to finite temperature, which requires the calculations of the Debye temperature Od from cjj in the case of no symmetry. The Debye theory is modified by introducing a parameter B2 that gives the fraction of the full Debye zero-point energy possessed by the actual dispersive mode frequencies. The value of the lattice parameter of fcc Al at low temperatures,a(T) , is shown to be accurately determined by the modified Debye theory of lattice vibrations and first-principles total energy band calculations with the MNP procedure. (3) The existence of structural anomalies in hcp Zn and Cd has been shown from first-principles total-energy calculations using WIEN2k with the EBP procedure. / Evaluation of the pressure dependence of various elastic quantities which are much more sensitive to the anomaly shows that the anomalies in hcp Zn and hcp Cd exist over a considerable range of pressure; several abrupt changes in the electron distribution are thereby indicated in that pressure range. (4) Calculations on the zone-center transverse optical phonon frequencies Vto(p) of hcp Zn, which found oscillatory behavior of Vto(p) in the pressure range of the anomalies, support the conclusions made in (3) on the structural anomalies. Based on this dissertation research four papers have been published in refereed journals. / by Florin Apostol. / Thesis (Ph.D.)--Florida Atlantic University, 2008. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2008. Mode of access: World Wide Web.

In the context of sustainable development, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as the present TRIP steels in order to reduce the fuel consumption and emission of greenhouse gas. These multiphase steels contain some retained austenite, a ductile phase that can transform into hard and brittle martensite during a mechanical solicitation. One the one hand, this transformation improves the mechanical properties during plasticity by bringing about an additional work-hardening. On the other hand, the appearence of a hard and brittle phase can give rise to premature cracking after necking. Knowing the good influence of martensitic transformation on the work-hardening, this Thesis starts with the characterisation of the relationship between transformation rates and testing conditions. It appears that, for each testing condition, there is an optimum austenite stability that leads to a maximum uniform strain. After necking under monotonic loading conditions, the damage mechanisms that takes place in these steels has been characterised. It can be concluded that the TRIP-aided steels that present low or moderate austenite stability behave exactly like Dual-Phase steels, in which martensite replaces retained austenite. However, a very stable retained austenite brings about a significant toughness improvement by providing an additional work-hardening contribution in the necking zone. The mechanical behaviour of these steels has also been characterised under cyclic loading conditions. The results indicate that, for particular loading conditions (i.e. low load levels), the martensitic transformation improves the fatigue properties.

No description available.

Aluminum and its alloys are used in many aspects of modern life, from soda cans and household foil to the automobiles and aircraft in which we travel. Aluminum alloy systems are characterized by good workability that enables these alloys to be economically rolled, extruded, or forged into useful shapes. Mechanical properties such as strength are altered significantly with cold working, annealing, precipitation-hardening, and/or heat-treatments. Heat-treatable aluminum alloys contain one or more soluble constituents such as copper, lithium, magnesium, silicon and zinc that individually, or with other elements, can form phases that strengthen the alloy. Microstructure development is highly dependent on all of the processing steps the alloy experiences. Ultimately, the macroscopic properties of the alloy depend strongly on the microstructure. Therefore, a quantitative understanding of the microstructural changes that occur during thermal and mechanical processing is fundamental to predicting alloy properties. In particular, the microstructure becomes more homogeneous and secondary phases are dissolved during thermal treatments. Robust physical models for the kinetics of particle dissolution are necessary to predict the most efficient thermal treatment. A general dissolution model for multi-component alloys has been developed using the front-tracking method to study the dissolution of precipitates in an aluminum alloy matrix. This technique is applicable to any alloy system, provided thermodynamic and diffusion data are available. Treatment of the precipitate interface is explored using two techniques: the immersed-boundary method and a new technique, termed here the sharp-interface method. The sharp-interface technique is based on a variation of the ghost fluid method and eliminates the need for corrective source terms in the characteristic equations. In addition, the sharp-interface method is shown to predict the dissolution behavior of precipitates in aluminum alloys when compared with published experimental results. The influence of inter-particle spacing is examined and shown to have a significant effect on dissolution kinetics. Finally, the impact of multiple particles of various sizes interacting in an aluminum matrix is investigated. It is shown that smaller particles dissolve faster, as expected, but influence the dissolution of larger particles through soft-impingement, even after the smaller particles have disappeared.

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This thesis is about the phase transformation, shape, size distribution and coalescence of TiO2 (part I) and ZrO2 (part II) nanopartilces produced by Nd-YAG laser ablation on metal targets under oxygen background gas, and characterized by analytical electron microscopy. The optimum laser ablation condition that satisfactory and routinely yield high-pressure phases of TiO2 (i.e. £\-PbO2-type and fluorite-related structures) and ZrO2 with high residual stress were reported. Part I-1 focuses on physical coagulation, by Van der Waals force, of the TiO2 condensates at temperatures up to about 1000 K as a result of post-condensation radiant heating. In part I-2, imperfect oriented attachment of nanoparticles over specific surfaces is rationalized to cause accretion and defects for the rutile condensates. Brownian motion may proceed above a critical temperature for anchorage release at the interface of imperfect attached nanoparticles until an epitaxial relationship is reached. Part I-3 deals with further the Brownian-type rotation of the imperfectly impinged £\-PbO2-type TiO2 and rutile nanocondensates until interfacial-energy cusp was reached. In part I-4 laser ablation condensation synthesis of dense TiO2 polymorphs and their phase transformations were documented. Part II-1 is about dense tetragonal (t)-ZrO2 and cubic (c-) nanocondensates which were synthesized under very rapid heating and cooling by pulsed Nd-YAG laser ablation with oxygen background gas. The t-ZrO2 nanoparticles were found to form deformation twins/faults and followed unique transformation path upon local electron dosage. Electron diffraction indicated that the dense c- and t- phase with specific size and residual stress were allowed to relax and/or kinetically phase change into lower-energy state as constrained by the intersections of the internal energy vs. cell volume plots calculated for the two polymorphs (Part II-2).